1. Field of the Invention
The invention relates to a digital duplex communication system comprising a transmitter/receiver including an echo canceller, the echo canceller comprising a memory unit having a plurality of locations for storing echo replicas, an address generator for causing echo replicas to be read out from selected locations of the memory unit depending on the transmitted data, a subtractor for subtracting the echo replica from the received signal, and adaption means for updating the stored echo replica.
2. Description of the Prior Art
Such a system is disclosed in U.K. Patent Application No. 2007946A. The memory unit in the echo canceller described in that application comprises a plurality of locations, the number of locations being equal to the number of possible combinations of transmitted data during the period over which the correction for each data bit is effective. It is suggested in the aforementioned patent application that the number of storage locations may vary from 4 to 128 and that a practical figure is 64. This assumes that for practical lengths of lines when using a bi-phase signal two to four biphase periods should be taken into account and the number of sampling points per bi-phase period at which correction is required may be from one to eight.
It will be apparent that any increase in the number of periods to be taken into account or in the number of points within a period to be taken into account will cause a corresponding increase in the required capacity of the memory unit.